The present invention relates to magnetic flowmeters of the type used to measure flow of process fluid. More specifically, the present invention relates to diagnostics of such flowmeters.
A magnetic flowmeter measures the volumetric flow rate of a conductive fluid by detecting the velocity of the fluid passing through a magnetic field. Magnetic flowmeter systems typically include a flowtube assembly and a transmitter assembly. The flowtube assembly is installed in a process piping line, either vertically or horizontally, and includes a pipe section, a coil section and electrodes. The coils are located on opposite sides of a cross section of the pipe. The coils, energized by a coil drive current from the transmitter, develop a magnetic field along the cross section of the pipe. Two electrodes are located across the pipe from each other generally along a line which is perpendicular to the magnetic field. Fluid passing through the pipe is electrically conductive. As a result of the conductor movement through the magnetic field, an electric potential or electromotive force (EMF) is induced in the fluid which can be detected across the electrodes. Operation is thus based on Faraday's law of electromagnetic induction.
Various types of failures can occur in a magnetic flowmeter which can lead to a current path which is in parallel with the coils. This parallel current path can cause some of the electric current to be shunted away from the coils, thereby leading to a reduced magnetic field applied to the process fluid. This reduced magnetic field will result in a reduced EMF being detected by the sense electrodes. The flow will be low by the amount of current lost. There are various situations which may cause such parallel current paths to arise. For example, the coils in the magnetic flowmeter flowtube may be compromised by process fluid leaking into the coil compartment. This can cause an electrical path in parallel with the coils. Similarly, if one of the transistors which are used to switch current through the coils fails, it may cause some of the coil current to be shunted past the coil through a parallel path. This leads to a reduction in the applied EMF and a corresponding reduction in the output from the sense electrodes. This will lead to inaccurate flow measurements.
Failures in components used to apply the coil drive signal, as well as failures or degradation in the current path of the coil drive signal, can reduce the magnetic field applied to the process fluid. This can lead to errors in flow measurements. Various diagnostic techniques have been used to detect such problems. For example, U.S. Pat. No. 7,921,733 entitled MAGNETIC FLOWMETER WITH COIL GROUND PATH DETECTION to Foss et al., issued Apr. 12, 2011 and assigned to Rosemount Inc. describes a particular diagnostic technique.